The disclosure relates to a catadioptric projection objective for imaging an object field arranged in an object surface of the projection objective onto an image field arranged in an image surface of the projection objective.
Catadioptric projection objectives are, for example, employed in projection exposure systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh resolution on a reduced scale.
In order to create even finer structures, it is sought to both increase the image-side numerical aperture (NA) of the projection objective and employ shorter wavelengths, preferably ultraviolet light with wavelengths less than about 260 nm. However, there are very few materials, in particular, synthetic quartz glass and crystalline fluorides, which are sufficiently transparent in that wavelength region available for fabricating the optical elements. Since the Abbe numbers of those materials that are available lie rather close to one another, it is difficult to provide purely refractive systems that are sufficiently well color-corrected (corrected for chromatic aberrations).
In optical lithography, high resolution and good correction status have to be obtained for a relatively large, virtually planar image field. It has been pointed out that a difficult requirement that one can ask of any optical design is that it have a flat image, especially if it is an all-refractive design. In general, providing a flat image requires opposing lens powers and that leads to stronger lenses, more system length, larger system glass mass, and larger higher-order image aberrations that result from the stronger lens curvatures.
Concave mirrors have been used for some time to help solve problems of color correction and image flattening. A concave mirror has positive power, like a positive lens, but the opposite sign of Petzval curvature. Also, in general, concave mirrors do not introduce color problems. Therefore, catadioptric systems that combine refracting and reflecting elements, particularly lenses and at least one concave mirror, are primarily employed for configuring high-resolution projection objectives of the aforementioned type.
Unfortunately, a concave mirror can be difficult to integrate into an optical design, since it sends the radiation right back in the direction it came from. Intelligent designs integrating concave mirrors without causing mechanical problems or problems due to beam vignetting or pupil obscuration are desirable.
Due to the increasing demands on the efficiency of the lithographic manufacturing process there is a tendency to increase the power of the light sources. Also, progressively shorter wavelengths are used. Specific illumination settings are employed to optimize the imaging conditions for various pattern types. As a result, various time-dependent changes in the properties of optical materials within the projection system are observed, which sensibly affect the imaging quality of the exposure system. The heating of lens groups and other transparent optical elements (“lens heating”) during operation due to an increased absorption is one effect dynamically influencing the imaging properties. Also, long-term (quasi-static) effects due to radiation induced refractive index variations, such as compaction effects, are often observed.
Applicant's patent application US 2004/0144915 A1 demonstrates an approach to solve some problems caused by absorption induced heating effects in a catadioptric projection objective having a physical beam splitter. The application discloses a folded catadioptric projection objective designed as a single imaging system (without intermediate image), where a concave mirror is positioned at the pupil surface. A physical beam splitter with a polarization-selective beam splitter surface is provided to separate radiation coming from the on-axis object field towards the concave mirror from radiation reflected by the concave mirror towards the image surface. The concave mirror is constructed as a deformable mirror, where the shape of the concave mirror surface can be manipulated using a pupil mirror manipulator in a manner allowing to compensate certain time-dependent imaging aberrations evolving during operation of the projection objective in response to radiation induced changes of the optical performance. The pupil mirror manipulator has simple construction and is installed at the back side of the concave mirror without interfering with the optical path. The deformable pupil mirror is designed to compensate, e.g., for astigmatism, two-fold or four-fold wavefront deformations due to absorption-induced heating of the cubic beam splitter and rectangular retardation plates, and for compaction effects and the like.